The present invention relates to an electromagnetic force-transducer, and more particularly to an electromagnetic force-transducer suitable for use in an electronic balance.
Electromagnetic force-transducers, particularly those used in electronic balances consist of a magnetic circuit and an electromagnetic coil kept movable in the static-magnetic field provided by said magnetic circuit, said electromagnetic coil being supplied with a current to produce an electromagnetic force. An example of such electromagnetic force-transducers is disclosed in the U.K. patent application GB 2 076 543 A.
FIG. 1 shows a cross-sectional view of a conventional electromagnetic force-transducer. In the figure a permanent magnet 1, a pole piece 2 and a flat-bottomed cylindrical yoke 3 constitute a magnetic circuit. The cylindrical yoke 3 has, on its inner surface, an annular prominence providing a pole piece section 3a which faces the pole piece 2 with an annular magnetic gap left therebetween. In the annular magnetic gap there is positioned an electromagnetic coil 4 wound around a coil frame 5 kept movable in a vertical direction coaxially with the magnet 1, the pole piece 2 and the cylindrical yoke 3. The coil frame 5 is to be provided with a force-transmitting means (not shown in the figure) connected to a force-loading portion (for instance, the weighing tray of an weighing balance) where a force to be balanced is loaded. The electromagnetic coil 4 supplied with a current, produces an electromagnetic force. With the current controlled suitably the electromagnetic force is enabled just to balance a force externally loaded to the coil 4. The magnitude of the electromagnetic force is obtained from measuring the current flowing in the coil 4. Therefore, it is desirable for the electromagnetic force to be exactly proportional to the current. However, the linearity between the current and the produced electromagnetic force is violated by the effect of the magnetic field which the coil 4 itself makes on the permanent magnet 1. A compensating coil 6 wound around the permanent magnet 1 is for cancelling the field made by the coil 4. The compensating coil 6, which does not contribute to the electromagnetic force production at all, is an obstacle to designing the force-transducer to be short in height. Further it makes the electric circuit complex.
On the other hand another type electromagnetic force-transducer is proposed, for instance, in the Japanese Laid-Open Utility Model Application No. 55-164519. In this model the area of the electromagnetic coil plane being divided into two halves bounded by a diameter, two magnetic fields directed oppositely to each other are applied respectively to one of the two halves and to the other half in the direction perpendicular to the coil plane so as to make the coil produce an electromagnetic force with its direction lying in the coil plane. With this manner of field application the effect of the magnetic field made by the electromagnetic coil is cancelled throughout the entire magnetic circuit so as to enable the produced electromagnetic force to be proportional to the current flowing in the coil. The force-transducer of this type, however, has a disadvantage that the effective circumferential length of the electromagnetic coil is reduced by a factor of 2/.pi..